Magnetic recording media for high density recording have been developed, and thus better surface smoothness of the magnetic layer of the magnetic recording medium is required to reduce space loss between a magnetic head and a magnetic recording tape. For this purpose, it is necessary to improve the surface smoothness of a magnetic layer. The surface smoothness of a magnetic layer can be improved by improving methods for producing magnetic recording media, for example, with respect to methods for dispersing the magnetic particles, coating methods, surface molding methods, and by improving surface smoothness of a support thereunder. Also, attempts have been made to make magnetic recording layers thinner in order to reduce loss which is caused due to the thickness of the layer, because the recording wavelength becomes shorter with increases in the recording density. Accordingly, higher surface smoothness of the support is required as the magnetic layer provided thereon is made thinner.
However, increasing the surface smoothness of a support which is used for a magnetic recording medium is limited for several reasons. Particularly, if the surface smoothness of the support is very high, it does not move smoothly in the step of winding it which is conducted after a film-coating step (e.g., calendering), and wrinkles tend to be generated on the film, because the frictional resistance of the film to rollers used for transporting the film is increased. Also, the shape of the wound film is not uniform, because frictional resistance between opposite surfaces of the rolled film is increased.
Various methods have been proposed to overcome the foregoing problems. For example it is disclosed in Japanese Patent Application (OPI) No. 109,605/83 (the term "OPI" as used herein means a "published unexamined Japanese Patent Application) that fine particles of thermoplastic resins which are present on a support can be removed by dissolving them with a solvent, after the support is wound up, and then a magnetic layer is coated on the support. However, even this method has not been able to provide fully satisfactory characteristics for a magnetic recording medium to be used for high density recording.
To solve the above problems, it has been proposed by the inventors of the present invention, as disclosed in Japanese Patent Application No. 187519/83 (corresponding to U.S. patent application Ser. No. 657,991 filed on Oct. 5, 1983) that an intermediate layer containing a compound which is polymerizable by radiation exposure be provided on a support having a surface roughness of 0.01 .mu.m or more, and be exposed to radiation for polymerization, followed by providing a magnetic layer thereon. Accordingly, even though a support having a high surface roughness is used, by providing a intermediate layer having a good surface smoothness, a magnetic layer having a good surface smoothness can be provided. However, when a magnetic layer is subjected to heat treatment for hardening, the surface of the magnetic layer is generally deteriorated. The inventors have made extensive studies on this problem and have found that the deterioration of the surface of the magnetic layer is due to the back surface of the support when the magnetic layer is subjected to heat treatment for hardening. That is, heat curing treatment is generally conducted, for example, at 60.degree. C. to 80.degree. C. for several days. The support thus provided with the foregoing intermediate layer and the magnetic layer thereon is wound up in a rolled state after the magnetic layer is dried, and is subjected to heat treatment in a rolled state for a long period of time. Therefore, the surface roughness on the back surface of the support transfers to the surface of the magnetic layer in contact therewith, and the surface smoothness of the magnetic layer is deteriorated.